The sulfonated amines of the invention have the formula- ##STR1## where R is H, CH.sub.3, C.sub.2 H.sub.5, halogen or ##STR2## x is the integer 0 to 2 and are economically important as intermediates in the manufacture of such varied products ranging from dyestuffs to sulfa drugs.
The more commonly used and thus economically important sulfonated amines are those derived from aniline, the toluidines, xylidines, chloroanilines, naphthylamines, aminoanthraquinones and the like.
The simplest of the sulfonated aromatic amines is sulfanilic acid of the formula: ##STR3##
Large amounts of sulfanilic acid are consumed annually, primarily to produce dyestuffs but some is also used in the synthesis of other materials including substantial amounts of phenylhydrazine-p-sulfonic acid.
Upon diazotization, diazobenzene sulfonic acid is formed. This is an important intermediate in the dyestuff industry.
The major portion of the sulfanilic acid is consumed in the manufacture of fluorescent whitening agents. Such fluorescent dyestuffs are applied to paper, woven and non-woven textiles, and are components of soap and detergent laundering agents. Such fluorescent whitening agents are marketed under the Tinopal trademark. For example, major amounts are used for the manufacture of one of the whiteners applied to paper. Thus, any savings effected in the production of sulfanilic acid are of major economic interest.
The most straight forward synthesis of these amines and particularly sulfanilic acid consists of reacting the amine, aniline, with sulfuric acid in substantially stoichiometric proportions to form aniline hydrogen sulfate (AHS), followed by splitting off water and the migration rearrangement of the resulting sulfonic acid group to the para position according to the following equations: ##STR4##
These reactions have been carried out directly or in the presence of inert, high boiling, water-immiscible solvents (Jacobs: Ind. Eng. Chem. 35; pg. 321-323).
The synthesis using such solvents produce a product of good appearance (Gardner color of 17 wt % solution of sodium salt&lt;9) but solvent cost, handling, removal and recycling impose excessive costs that are not competitively bearable. Thus most sulfanilic acid until now was prepared in the absence of solvents.
The reaction (I) between aniline and sulfuric acid to form the aniline hydrogen sulfate proceeds as rapidly as the reactants are mixed and is exothermic. The heat of the exothermic reaction causes the temperature of the reaction mass to rise. Aniline hydrogen sulfate melts at about 160.degree. C. It is very soluble in water at elevated temperatures. The temperature at which it liquifies varies with the amount of water in the mixture.
To cause water to be split off and to promote the para-rearrangement of reaction (II) at a slow but measurable rate, the minimum temperature must be about 165.degree. C.-170.degree. C. At this temperature, the aniline hydrogen sulfate is slowly converted, by an endothermic reaction, to sulfanilic acid. Sulfanilic acid, (in contrast to the aniline hydrogen sulfate salt) is stable and does not melt up to its decomposition temperature which is about 280.degree. C.
It is instructive to visually observe the solventless conversion to sulfanilic acid of a molten mass of aniline hydrogen sulfate. As stated above, when water is present in the aniline hydrogen sulfate, the mixture forms a liquid or pasty mass at temperatures below 160.degree. C. As the mass is heated, water is driven off so that substantially none is present at about 160.degree. C.
When the temperature is raised to the rearrangement temperature, the conversion starts.
Sulfanilic acid is insoluble in molten aniline hydrogen sulfate. During the early stages of the conversion the molten mass becomes a pasty, sticky mass of a liquid phase and a dispersed solid phase. Bubbles form in the liquid from the water vapor that evolves. As the conversion continues, the mass becomes increasingly solid with a liquid phase dispersed within it. The upper surface is no longer smooth. It assumes a pockmarked appearance from the simultaneous escape of water vapor and the solidification of that part of the mixture from which it has emerged.
By maintaining the sample above the conversion temperature for a sufficient period of time, the sample is substantially completely converted to a block of solid sulfanilic acid which strongly adheres to the surface with which it was in contact during the conversion. It is this behavior pattern of the mixture during its conversion that has led to the previously employed dry processes briefly described below.
During this period of dry rearrangement and water elimination, color bodies of indeterminate composition form; causing a grayish or deep purplish product (Gardner color of 17 wt % solution of 8-9). For many uses this product, if not too intensely colored, can be used without further upgrading. It is known that iron tends to catalyze a reaction which also forms color bodies in the product.
In the dry processes, aniline and sulfuric acid, in substantially equimolar amounts are charged to the reactor to form aniline hydrogen sulfate therein. When the charging is completed, the temperature within the surrounding furnace is raised to heat the reactor and its contents above the rearrangement temperature. The released water is vented and is externally condensed.
Dry rod mill reactors and the tunnel kiln processes commonly used, are basically batch processes. The reactors have to be heated and then cooled. Heat transfer through the solid and semi-solid phases is poor. Rod mill reactors also serve as a mill to break up the lumps and break off the scale, thereby promoting the production cycle.
Other non-solvent "dry" processes include forming a pool of molten aniline hydrogen sulfate in which a segment of a continuously rotating drum dryer is immersed. The interior of the dryer is heated by high pressure steam or some other heat transfer fluid. The aniline hydrogen sulfate adhering to the exterior surface of the drum, after it emerges from the pool, is thus heated above the rearrangement temperature. The speed of rotation of the drum and the temperature to which the adherent film is raised are correlated so that rearrangement to sulfanilic acid is complete and the product removed before reentry of the drum portion into the molten pool. While feasible, the parameters of temperature, rotation rate, product adherence, separation, etc., render production control difficult so that this process is not economically viable.
Another "dry" method is based on mixing sulfuric acid, aniline and a small amount of water at about 125.degree.-145.degree. C. and spraying the resulting solution into a spray dryer along with air heated to about 400.degree. C. The temperature of the outgoing air is held at about 260.degree.-270.degree. C. The resulting air stream, in which solid sulfanilic acid particles are entrained, is passed through a cyclone and bag filters. This separates the air from the solids. This method requires completion of the rearrangement in a very short period--in the order of a second. Current environmental requirements are very stringent and to meet even minimal solid separation requirements of the solid product from the gases necessitates inordinately expensive arrangements and investments.
Another "dry" variant involves utilization of fluidized bed technology wherein either solid or molten aniline hydrogen sulfate is continuously fed to a fluidized bed of sulfanilic acid. The bed is fluidized by a sufficiently hot inert gas to maintain the bed above the rearrangement temperature. The water vapor, which evolves, leaves carried by the vented inert fluidizing gas. Sulfanilic acid is continuously or periodically withdrawn from the reaction bed. The inert gas, which may be heated air at about 200.degree. C.-250.degree. C., is not only used as a fluidizing medium but also as the heating element and the carrier for sweeping out the water. Of course, this method requires very efficient cyclones and/or bag filters and often porous metal filters fitted with cyclic filter cleansing means to remove the product from the vented gases. These are expensive so that this method is non-competitive with the batch rod mill method.
All the above dry processes produce sulfanilic acid and/or aromatic amine p-sulfonates of considerable color. These colored products usually need purification to remove the undesirable colored intermediates which affect the purity of further products prepared therewith. The steps of decolorization by carbon absorption and other expedients raise the final cost of these important intermediates, particularly sulfanilic acid.
It is an object of this invention to prepare the above described sulfonated amine, at reasonable cost, at high purity and in good yield.
It is among further objects to provide a process for making such sulfonated amines of good color (about Gardner 1.0) by a process which is ecologically viable.
It is also an object to provide a process which permits a complete recycle of initially unutilized reactants- thus no effluent is produced and all the available energy produced by the reaction steps is absorbed in forwarding the reaction.
Another object is to provide for the preparation of the sulfonated amine, sulfanilic acid, of excellent color and at competitive costs by eliminating costly decolorizaiton steps.